Microflora estafilocócica de leche natural

Abstract

Studies have been carried out on 86 samples of raw milk, 50 taken from the reception department of a dairy industry and the rest from three milking-sheds in the province of León. After a preliminary study of the properties of each sample (pH, total acidity, temperature, total number of microorganisms, staphylococci, probable positive coagulase staphylococci, and the verification, in 24 samples, of some staphylococci which survived a temperature of 71.5° C for a period of 16 seconds) the isolation of the distinguishable strains was carried out according to the different morphologies of the colonies. A total of 208 staphylococci were isolated, of which 58 proved to be coagulase positive. The isolated microorganisms have been amply characterized from the point of view of physiology, biochemistry and toxicity (verification of hemolysins and enterotoxin, but the latter in only 18 coagulase positive staphylococci). At the same time the coagulase positive microorganisms were submitted to phage typing (INTERNATIONAL SERIES and SETO-WILSON) and to antibiotic action in order to establish the proportion of staphylococci of milk origin resistant to these drugs. For the purpose of this study a new medium of selective culture (MAF) was used, based on the growth of staphylococci in anaerobic and semianaerobic conditions and the use of mannitol as a source of carbon in these circumstances. Due to the favourable results obtained with this medium, and also with the variation MAF-2, using egg-yolk, we propose its use as a new technique for the isolation of staphylococci of milk origin. From the results obtained in the preliminary study of the samples (all with minimum differences in acidity) it is deduced that the correlation intensity between some qualities of the samples is not as high as was to be expected if the principles on which these properties depend are taken into consideration (increase in acidity due to the fermentation of the lactose produced by lactic bacteria, and at the same time in relation with the temperature, number of microorganisms, etc.) underlining in this way the role of other milk qualities which are normally considered secondary (initial bacteriostatic power, buffer effect, fodder influence in its acidity, etc.). Enzymatic characterization has enabled a comparison of the tests used, underlining the fermentation of mannitol, phosphatase and desoxyribonuclease production as the most outstanding to determine the pathogenic capacity of staphylococci deduced in this case from its ability to coagulate rabbit and human plasma. Enterotoxin production was checked by inoculation in cats and was confirmed by gel-diffusion techniques in the 5 positive cases of the 18 tested. Experiments using the histamine valuation technique on an isolated organ (guinea pig ileum) and on cats' blood pressure, to diagnose enterotoxin, gave no practical result, but it was noticed, in all the microorganisms of complex enzymatic qualities tested, a light but persistent action, in the culture filtrates, on the guinea pig ileum submerged in a nutritious liquid. This action was of an antihistaminic nature. The phages which most frequently affected staphylococcus strains which were submitted to test were the 42E, 42D, 83A, 54 and 81 of the INTERNATIONAL SERIES and the S2, S5 of the SETO-WILSON SERIES. The signs of resistance to antibiotics in staphylococci of bovine origin are surprisingly high (22.2 % of the coagulase positive microorganisms) in a country in which the use of antibiotics in bovine medical treatment is not yet widely extended